# Subjective expected utility

Subjective expected utility is a method in decision theory in the presence of risk, promoted and axiomatized by L. J. Savage in 1954[1] [2] following previous work by Ramsey and von Neumann.[3] The theory of subjective expected utility combines two subjective concepts: first, a personal utility function, and second a personal probability distribution (usually based on Bayesian probability theory).

Savage proved that, if you adhere to axioms of rationality, if you believe an uncertain event has possible outcomes $\{x_i\}$ each with a utility to you of $u(x_i)$ then your choices can be explained as arising from a function in which you believe that there is a subjective probability of each outcome is $P(x_i)$, and your subjective expected utility is the expected value of the utility,

$\sum_i \; u(x_i) \; P(x_i).$

You may be able to make a decision which changes the possible outcomes to $\{y_j\}$ in which case your subjective expected utility will become

$\sum_j \; u(y_j) \; P(y_j).$

Which decision you prefer depends on which subjective expected utility is higher. Different people may make different decisions because they may have different utility functions or different beliefs about the probabilities of different outcomes.

Savage assumed that it is possible to take convex combinations of decisions and that preferences would be preserved. So if you prefer $x(=\{x_i\})$ to $y$ and $s$ to $t$ then you will prefer $\lambda x + (1-\lambda )s$ to $\lambda y + (1-\lambda )t$, for $0<\lambda<1$.

Experiments have shown that many individuals do not behave in a manner consistent with Savage's axioms of subjective expected utility, e.g. most prominently Allais (1953)[4] and Ellsberg (1961).[5]

## Notes

1. ^ Savage, Leonard J. 1954. The Foundations of Statistics. New York, Wiley.
2. ^ Karni, Edi. "Savage's subjective expected utility model." The New Palgrave Dictionary of Economics. Second Edition. Eds. Steven N. Durlauf and Lawrence E. Blume. Palgrave Macmillan, 2008. The New Palgrave Dictionary of Economics Online. Palgrave Macmillan. 23 August 2014 <http://www.dictionaryofeconomics.com/article?id=pde2008_S000479> doi:10.1057/9780230226203.1474
3. ^ Ramsey says that his essay merely elaborates on the ideas of Charles Sanders Peirce. John von Neumann noted the possibility of simultaneous theory of personal probability and utility, but his death left the specification of an axiomatization of subjective expected utility incomplete, until Johann Pfanzagl's work.
4. ^ Allais, M. (1953). "Le Comportement de l'Homme Rationnel Devant Le Risque: Critique des Postulats et Axiomes de L'Ecole Americaine". Econometrica 21 (4): 503–546. doi:10.2307/1907921.
5. ^ Ellsberg, Daniel (1961). "Risk, Ambiguity and Savage Axioms". Quarterly Journal of Economics 75 (4): 643–79. doi:10.2307/1884324.

## References

de Finetti, Bruno. "Foresight: its Logical Laws, Its Subjective Sources," (translation of the 1937 article in French) in H. E. Kyburg and H. E. Smokler (eds), Studies in Subjective Probability, New York: Wiley, 1964.